1. Field of the Invention
The present invention relates to a sheet conveying apparatus for conveying a sheet material by attraction to a conveyor belt, and an image forming apparatus equipped with such sheet conveying apparatus.
2. Related Background Art
An image forming apparatus such as a printer, a copying apparatus or a facsimile apparatus forms an image constituted of dot patterns, according to image information, on a sheet material (recording medium) such as paper or a plastic thin plate. Such image forming apparatus can be classified, according to recording methods, into an ink jet type, a wire dot type, a thermal type, a laser beam type and the like, and the apparatus of the ink jet type is so constructed as to form an image by discharging ink from a recording head onto a sheet material such as recording paper.
In the image forming apparatus, the sheet material has to be conveyed from a sheet feeding portion such as a cassette, through an image forming portion (recording portion) to a sheet discharge portion. The conveying operation of the sheet material after the sheet feeding is controlled at constant timings in the course of image formation until the sheet is discharged. Within such process, a particularly accurate conveying operation is required from the sheet feeding to the image formation, as the image forming position on the sheet material is affected. Also in the course of image formation, the conveying speed of the sheet material has to be constant, as otherwise an image magnification may be affected to result in an elongation or a contraction of the image. Particularly in case of a color image forming apparatus utilizing plural recording heads, images recorded in respective image forming portions are mutually displaced, and such displacement results in a color aberration in the color image forming apparatus, thus giving rise to an image defect. In order to avoid such drawback, it is necessary to accurately transmit a conveying force of precisely controlled conveying means to the sheet material.
In consideration of the foregoing situation, various conveying means have been proposed. For example, there is known a method of advancing a sheet material by a pair of rollers and limiting a conveying direction by a guide member. As the sheet material is advanced by a pressure between the rollers, this method has a strong conveying power and is therefore simple and secure. However, the paired rollers have to be positioned at an interval shorter than a smallest length of the sheet material to be used, and is unsuitable for conveying a sheet material of a size of a postcard or a name card. Also such method cannot be adopted in case a recorded surface has to be maintained in a non-contact state, as in a path from an image transfer, from an electrophotographic drum, to an image fixation.
In order to avoid such drawback, there is proposed a conveying apparatus utilizing an endless belt and attracting the sheet material to such endless belt by electrostatic attraction. In such conveying apparatus of electrostatic attraction belt type, particularly in case of a color image forming apparatus utilizing plural recording heads (image forming means), the conveying speed of the belt has be maintained exactly in order to obtain an exact image forming position by each recording head. In addition, the sheet material has to be maintained in close contact with the conveying member (belt or drum), without displacement or turn-up. For this purpose, Japanese Patent Application Laid-Open No. S50-031828, Japanese Patent Publication No. H03-048100 and Japanese Patent Application Laid-Open No. 2000-247476 disclose a method of providing a conveying belt, for supporting a recording paper, with a pair of conductive electrodes (attraction force generation means) and providing a charge to generate an electrostatic force thereby attracting the recording sheet.
In an ink jet recording apparatus equipped with such conveying means, a recording sheet supplied from a sheet feeding apparatus is, in a recording area, attracted and supported on the surface of the conveying belt by such electrostatic attraction means (conductive electrodes), and is conveyed under a recording by the recording head. A representative structure is described in Japanese Patent Application Laid-Open No. 2000-247476. FIGS. 27 and 28 are respectively a schematic plan view and a schematic lateral view, illustrating a schematic structure of a conveying apparatus having such conveying belt.
Referring to FIGS. 27 and 28, an endless conveying belt 101 is supported around a drive roller 102, a conveying roller 103, and a pressure roller 104 so biased as to maintain the conveying belt 101 at a predetermined tension. A rotary driving power of a driving motor 105 is transmitted through a belt 106 to the driving roller 102, which thus rotates at a predetermined speed to cause the conveying belt 101 to run. A high voltage is applied from an electric power supply brush 107 to plural electrodes 110. The electric power supply brush 107 is given a positive or negative high voltage (electric power) from a high voltage generator 108.
FIG. 29 is a schematic partial plan view showing a part D1 in FIG. 27, and FIG. 30 is a schematic lateral view of the electric power supply brush 107 shown in FIGS. 27 to 29. FIG. 31 shows an electrode pattern (hatched portion), provided under a surface layer of the conveying belt 101. In FIG. 29, an electrode 110 is constituted of plural positive electrodes 110A and plural negative electrodes 110B. Each positive electrode 110A and each negative electrode 110B extend in a direction perpendicular to the conveying direction of the conveying belt 101, and are alternately provided at a predetermined pitch along the conveying direction of the conveying belt 101. The positive electrodes 110A and the negative electrodes 110B are set at a potential difference of several kilovolts. The negative electrode 110B may be connected to a ground potential or given a negative voltage, suitably selected according to conditions of the functions and performance required for the actual equipment.
The positive electrode 110A is provided, at an end portion in the direction of width of the conveying belt 101, with an electric power receiving portion 110A1, and the negative electrode 110B is provided, at an end portion in the direction of width of the conveying belt 101, with an electric power receiving portion 110B1. The voltage supply from the electric power supply brush 107 to the positive electrode 110A and the negative electrode 110B is executed through these electric power receiving portions 110A1, 110B1. Also these electric power receiving portions 110A1, 110B1 are so formed as to be reachable through apertures 101a, 101b formed in a surface layer of the conveying belt 101. A brush portion of the electric power supply brush 107 contacts the electric power receiving portions 110A1, 110B1 through these apertures 101a, 101b thereby executing an electric power supply to the electrode 110 (positive electrodes 110A and negative electrodes 110B). An area showing a potential difference between the positive electrode 110A and the negative electrode 110B under the electric power supply constitutes a attraction area 112, as illustrated in FIG. 27.
As the sheet material such as recording paper is a dielectric material such as paper, the sheet material supplied from an upstream side the conveying belt 101 in the conveying direction thereof and placed on the belt becomes polarized, upon reaching the attraction area 112, by an electric field generated in the electrode 110 constituting attraction force generation means. As a result, a mutually attracting electrostatic force is generated between the sheet material and the conveying belt 101, whereby the sheet material is attracted by and supported on the conveying belt 101.
However, the attraction force for the sheet material, generated by a comb-shaped electrode pattern as in the electrode 110, shown in FIGS. 27 to 30, is not necessarily distributed uniformly. A spatial distribution of the electric force lines is variable among an electrode-free portion, an end portion of the electrode and a central portion of the electrode, so that the attraction force is not constant. It is experimentally found that the attraction force is relative strong in an end portion of the electrode and is relative weak in a portion lacking the electrode. Therefore, the sheet material and the electrode should be provided in such a mutual positional relationship as to secure a sufficient attraction force in an end portion of the sheet material where the sheet material tends to be lifted.
On the other hand, in case the attraction force is deficient in an end portion of the sheet material conveyed on the belt, there may result a situation where the end portion of the sheet material is not in close contact with the conveying belt because of a deformation of the sheet such as a curl in the end portion. In case such phenomenon occurs in a situation, for example in an ink jet recording apparatus, where a structure or a mechanism such as a recording head is present closely above the conveying belt, the sheet material under conveying may contact such structure or mechanism. In case the sheet material under conveying comes into contact with such closely positioned structure, the sheet material rapidly loses planarity locally in such contact portion or entirely, thereby resulting in a conveying failure. Also a contact between the sheet material and the recording head may hinder the printing ability of the recording head.
Thus, as shown in FIGS. 27 to 30, the prior electrode pattern is formed in a direction perpendicular to the conveying direction of the sheet material, and no particular attention is paid between the positional relationship between a front end portion of the sheet material, supported on the running conveying belt, and the electrode. It is therefore not possible to suppress a turn-up in an end portion of the sheet material, and is defective in the conveying ability.
Therefore, Japanese Patent Application Laid-Open No. 2000-247476 (patent reference 1) discloses a conveying belt, provided with attraction force generation means constituted of an electrode pattern which is capable of generating a attraction force even for an end portion of the sheet material and which is less influenced by a direction of fibers in the sheet material. A belt conveying apparatus and an image forming apparatus utilizing such conveying belt have a following configuration. In a conveying belt including a belt member having a supporting surface for supporting a sheet material, and attraction force generation means which includes plural opposed electrodes provided in a attraction force generating area provided in a central portion in the width of the conveyed belt member, and an electric power receiving portion provided in at least an end portion of the belt member in the width of the conveyed belt member, for electric power supply to the opposed electrodes, the plural opposed electrodes include an area formed in inclined manner to the direction of width of the conveyed belt member.
Also Japanese Patent Application Laid-Open No. 2000-60168 (patent reference 2) discloses a configuration of a conveying belt for suppressing a turn-up of a front end portion of a sheet material. FIGS. 31 and 32 are schematic plan views showing two examples of such conveying belt. In FIGS. 31 and 32, in a conveying belt in which positive and negative voltages are alternately applied to electrode patterns formed with a constant gap on the surface of a conveying belt 10, thereby conveying a recording medium under an electrostatic attraction, each electrode is so constructed as to be inclined in zigzag manner to the direction of width of a substrate of the conveying belt.
However, the aforementioned prior technologies are associated with following drawbacks. In the technology of Japanese Patent Application Laid-open No. 2000-247476, a contact position and a attraction start position of the sheet material with the conveying belt are not specified, and, when a front end of the sheet material comes into contact with the conveying belt, a attraction force may be exerted over the entire front end. In such case, the attraction force is exerted in the entire contact area starting from the attraction start position, and the attraction force is exerted in the entire contact area even when the contact area increases under the running of the conveying belt. Under a change in environmental conditions such as temperature and humidity, the sheet material may come into contact with the conveying belt in a waved or curled state, and subjected to a attraction force. Thus, when the sheet material is attracted to the belt in a state deformed in a convex shape from the conveying belt, the attraction may take place in a state where air remains in a space between an internal surface of such convex shape and an upper surface of the conveying belt.
In case the sheet material is of a material enabling gas permeation such as a plain paper, the air eventually remaining at the start of attraction can escape through the surface of the sheet material under a attraction force between the conveying belt and the sheet material, thereby enabling a attraction over the entire area without remaining air. However, in case of a sheet material of low air permeation such as a photographic coated paper (Professional Photopaper PR-101, trade name of Canon Inc.), the air remaining between the conveying belt and the sheet material is incapable of passing through the sheet material and therefore remains between the conveying belt and the sheet material, whereby a portion of the sheet material, where the air remains, floats in a convex shape.
Also in Japanese Patent Application Laid-Open No. 2000-60168, a contact start position of the sheet material to the conveying belt, and an electrode position generating a attraction force under an electric power supply are not clearly defined. Therefore, the technology in Japanese Patent Application Laid-Open No. 2000-60168, as in Japanese Patent Application Laid-Open No. 2000-247476, may result in a situation where, in case of a sheet of low air permeation, air remains between the conveying belt and the sheet material to generate a convex floating portion therein, and such floating portion may contact a discharge surface of an ink jet head thereby causing a recording failure.
Also the sheet material conveying apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-247476 does not specify a contact position and a attraction start position of the sheet material with the conveying belt, nor a positional relationship of the electric power supply brush, nor is not suitable for compactization of the apparatus. Also Japanese Patent Application Laid-Open No. 2000-60168, not being clear, in the sheet material conveying belt (medium conveying base member), on a contact start position between the sheet material and the conveying belt and on a position and a range of electrodes generating a attraction force under an electric power supply, is not suitable for compactization of the apparatus, as in the case of Japanese Patent Application Laid-Open No. 2000-247476.